The corticospinal (CS) tract controls our skilled voluntary movements and its disruption leads to a debilitating life or even death. Proper CS tract formation requires the transcription factor FEZF2 (FEZL or ZFP312) in neocortical layer 5 neurons. In my K99 phase, 1 showed that FEZF2 controls the transcription of a layer specific combinatorial code required for semaphorin-mediated guidance of CS axons, which leads to proper formation of CS circuits, including CS thalamic collaterals. Remarkably, the inactivation of CS semaphorin guidance molecules led to changes in the regionally and temporally restricted transcriptional programs of layer 5 neurons. In particular, the onset of the expression of F0XP2 in layer 5 of sensorimotor neocortices depends on the semaphorin-dependent proper formation of thalamic CS collaterals and on proper thalamic neurotrophin signaling. In my ROO phase, I will continue my work on the central hypothesis that in addition to cell-intrinsic actions of Fezf2 and other related transcription factors, proper development of layer 5 CS neurons and its synaptic connections depends on the retrograde signaling from the thalamus, which occurs before finalization of CS development. Therefore, I will first identify other CS gene candidates to be connectivity dependant. Next, I will test the effect of these gene candidates in vivo on the CS formation and their possible role in CS regeneration. Finally, I will reveal CS axonal intracellular mechanisms of thalamus-dependant retrograde signaling. On a long-term goal, 1 aim to test for the alterations of thalamic dependant CS genes in distinct neurological and psychiatrical disorders. Thus, with this proposal, 1 aim to continue revealing molecular mechanisms of retrograde signaling on formation and molecular differentiation of the CS system and other neocortical circuits. Results should have a high impact on the understanding of sensorimotor integration and should open or extend horizons for novel treatment approaches for neurological and psychiatric disorders affecting the CS tract and the thalamus, and for regeneration of severed CS tract in adulthood.